Studies of bipolar nebulae in the central bulge of the Milky Way have revealed an unexpected alignment. The objects shown here are much closer to Earth than those used in the new study but demonstrate the varied forms of these spectacular objects. // ESO

Astronomers have used the European Southern Observatory’s (ESO) New Technology Telescope (NTT) and the Hubble Space Telescope to explore more than 100 planetary nebulae in the central bulge of our galaxy. They have found that butterfly-shaped members of this cosmic family tend to be mysteriously aligned — a surprising result given their different histories and varied properties.

The final stages of life for a star like our Sun result in the star blowing its outer layers out into the surrounding space, forming objects known as planetary nebulae that take a wide range of beautiful and striking shapes. One such type of nebula, known as a bipolar planetary nebula, creates a ghostly hourglass or butterfly shape around its parent star.

Bipolar nebulae form in different places and have different characteristics. Neither the individual nebulae nor the stars that formed them interact with other planetary nebulae. However, a new study by astronomers from the University of Manchester in the United Kingdom shows surprising similarities between some of these nebulae: many of them line up in the sky in the same way.

“This really is a surprising find and, if it holds true, a very important one,” explained Bryan Rees of the University of Manchester, one of the paper’s two authors. “Many of these ghostly butterflies appear to have their long axes aligned along the plane of our galaxy. By using images from both Hubble and the NTT, we could get a really good view of these objects, so we could study them in great detail.”

The astronomers looked at 130 planetary nebulae in the Milky Way’s central bulge. They identified three different types and peered closely at their characteristics and appearance.

“While two of these populations were completely randomly aligned in the sky, as expected, we found that the third — the bipolar nebulae — showed a surprising preference for a particular alignment,” said the paper’s second author, Albert Zijlstra, also of the University of Manchester. “While any alignment at all is a surprise, to have it in the crowded central region of the galaxy is even more unexpected.”

Scientists think planetary nebulae are sculpted by the rotation of their parent star system. Their shape, then, is dependent on the star system’s properties — for example, whether it is a binary or has planets, both of which greatly influence the form of the blown bubble. The shapes of bipolar nebulae are some of the most extreme.

“The alignment we’re seeing for these bipolar nebulae indicates something bizarre about star systems within the central bulge,” explained Rees. “For them to line up in the way we see, the star systems that formed these nebulae would have to be rotating perpendicular to the interstellar clouds from which they formed, which is very strange.”

While the properties of their progenitor stars do shape these nebulae, this new finding hints at another, more mysterious factor — one having to do with the larger galaxy. The whole central bulge of the Milky Way rotates around the galactic center. This bulge — specifically its magnetic fields — may have a greater influence than scientists previously thought over the entire galaxy. The astronomers suggest that the presence of strong magnetic fields as the bulge formed could have caused the orderly behavior of the butterfly planetary nebulae.

Bipolar nebulae that are closer to Earth do not line up in the same orderly way, so scientists postulate that these central magnetic fields were many times stronger than they are in our present-day neighborhood.

“We can learn a lot from studying these objects,” concludes Zijlstra. “If they really behave in this unexpected way, it has consequences for not just the past of individual stars, but for the past of our whole galaxy.”